The invention relates to a device and a method for orbit determination and prediction of satellites, particularly of navigation satellites.
Satellite systems for navigation (GNSS; GNSS=Global Navigation Satellite System) allow accurate determination of a position on Earth or in the air. GNSS, such as, for example, the European satellite navigation system Earth-based Galileo comprise a plurality of satellites and an Earth-based control system comprising several control stations.
The accuracy of a GNSS depends on several parameters, one of which is the quality of the knowledge of the orbit of each satellite. In order to determine and predict the orbit of a satellite, principal orbit modeling is usually performed by applying the theory of gravity and estimating non-gravity forces. It is difficult to model the non-gravity forces. While these forces can be empirically estimated for orbit determination using any kind of measurements, they significantly limit the accuracy in case of orbit prediction.
Non-gravity forces include, for example, solar radiation pressure, air drag of the Earth's atmosphere, thrusters forces, Earth infrared radiation, magnetic interaction with the Earth's magnetosphere and ionosphere. While the magnitude of these forces might be negligible for orbit propagation of many satellite missions, it is not for navigation satellites where a precise orbit prediction is essential.
In the state-of-the-art orbit determination, non-gravity forces are usually treated in a way that an empirical model is fit to the observation residuals after all gravity forces have been subtracted. The empirical model is usually defined in a way that a best fit is obtained and the remaining residuals are very small.
This, however, has the drawbacks that the empirical model is not at all related to the physical nature of the effects; the different non-gravity forces are not treated independently which makes a meaningful interpretation of respective results impossible; and, most significantly, an extrapolation of the empirical model parameters, which is necessary for orbit prediction, is poor and unstable.
Therefore, it is an object of the present invention to provide an improved device and an improved method for orbit prediction of satellites, particularly of navigation satellites.
One aspect of the present invention comprises the application of an accelerometer in a satellite, wherein the accelerometer is provided and adapted to measure the non-gravity accelerations acting on the satellite. The measurements may be used to estimate the contributions of non-gravity accelerations to the movement of a satellite, particularly during the orbit determination process. The accuracy which can be achieved for the acceleration measurements may be in the order of about 1E-8 to about 1E-9 with very limited effort. Even an accuracy of about 1E-10 might be reachable with acceptable effort. Thus, the orbit prediction may be significantly improved over the prior art.
According to an embodiment of the invention, a device for orbit prediction of satellites, particularly of navigation satellites, is provided, wherein the device comprises an accelerometer that measures non-gravity accelerations acting on a satellite, and a processor. The processor receives measured non-gravity accelerations from the accelerometer, estimates the contributions of the measured non-gravity accelerations to the movement of the satellite, and predicts the orbit of the satellite based on the estimated contributions of the non-gravity accelerations/forces to the movement of the satellite.
The precise measurements with the accelerometer and the decoupling from all gravity forces allow a very good estimation of the contributions of non-gravity forces. These estimated contributions may be used to improve the prediction of the satellite orbit.
The processing means may further estimate the contributions of the non-gravity forces to the movement of the satellite during an orbit determination process.
Furthermore, the device may receive satellite attitude data, and the processor determines artificial effects introduced by attitude maneuvers of the satellite based on the received satellite attitude data, and accounts for the determined artificial effects during the estimation of the contributions of the non-gravity forces to the movement of the satellite.
The satellite attitude data allows more precise processing of accelerometer measurements by including the satellite's attitude in the processing, particularly in order to remove artificial effects which are introduced by attitude maneuvers and may influence the accuracy of orbit prediction, especially when the accelerometer is placed off the center of mass of the satellite.
Those satellite attitude data are usually available from the satellite control agency. Thus, the device may also receive a signal containing the satellite attitude data from a satellite control agency. Then, the agency may prepare and transmit a signal containing satellite attitude date for correcting the satellite's orbit.
In case the accuracy of these data is not sufficient, the accelerometer might be combined with a gyroscope. Thus, the device may comprise a gyroscope that measures rotation forces acting on the satellite, and the processor receives measured rotation rates from the gyroscope, and determines artificial effects introduced by attitude maneuvers of the satellite based on the received measured rotation rates if the accuracy of satellite attitude data received from the satellite control agency is not sufficient. This allows to further improve the accuracy of orbit prediction particularly in the case of attitude maneuvers.
The device may also use the estimated non-gravity contributions to the satellite's movement determined during the orbit determination process for the orbit prediction and/or the generation of the navigation message for the user.
According to the invention, the processor of the device may be part of a satellite or part of one or several locations on ground.
The device may also compare the measured non-gravity forces with non-gravity forces used in the orbit prediction to generate the user message, and trigger an integrity alert for the ranging signals emitted by the satellite if the result of the comparison is larger than a predefined threshold. The integrity alert may be either autonomously sent by the satellite to the users or sent to the processing facility which incorporates this information in the integrity data flow.
Briefly summarized, measured non-gravity forces may be compared with the non-gravity forces used in the orbit prediction for generation of the user message of the satellite, and detected differences may be used to trigger integrity alerts. The integrity alerts may be, for example, received by a navigation device and used to warn a user of the navigation device about an inaccuracy in the navigation.
The invention also provides a navigation satellite used in a global navigation satellite system and comprising a device of the invention for accurately predicting the orbit of the navigation satellite. For example, the navigation satellite may be particularly adapted to be applied with Galileo.
The navigation satellite may autonomously signal to a user equipment a mis-fit of a maneuver prediction and execution, and send out integrity alert messages in case of mis-fits. This allows implementing a GNSS with a high degree of integrity since users may be warned about inaccurate navigation signals from each satellite. The integrity alert messages may be, for example, processed in a navigation device and cause a change of processing of satellite signals, for example, to switch to processing satellite signals from another satellite which did not send out integrity alert messages.
The invention also involves a method for orbit prediction of satellites, particularly of navigation satellites. The method includes measuring non-gravity accelerations acting on a satellite with an accelerometer, receiving measured non-gravity accelerations from the accelerometer, estimating the contributions of the measured non-gravity accelerations/forces to the movement of the satellite, and predicting the orbit of the satellite based on the estimated contributions of the non-gravity forces to the movement of the satellite.
A computer program may be provided, which is enabled to carry out the above method according to the invention when executed by a computer.
According to a further embodiment of the invention, a record carrier storing a computer program according to the invention may be provided, for example a CD-ROM, a DVD, a memory card, a diskette, or a similar data carrier suitable to store the computer program for electronic access and execution by a computer.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
The invention will be described in more detail hereinafter with reference to exemplary embodiments. However, the invention is not limited to these exemplary embodiments.
In the following, functionally similar or identical elements may have the same reference numerals. In the following description, absolute values may be contained. These absolute values are only exemplary values and do not restrict the scope of the present invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.